1. Field of Invention
The present disclosure relates to terahertz generation with lasers and, more particularly, to a high repetition rate terahertz generation system using a radio frequency bias.
2. Description of Related Art
Terahertz (THz) radiation technology is becoming increasingly popular. It has a number of applications in sensing and imaging. The power of THz radiation generated using photoconductors is also useful. It could be the basis for important advances in homeland security, defense and health care.
The term “terahertz radiation” may be used to refer to electromagnetic radiation falling in the very far- to mid-infrared spectral range, corresponding to frequencies ranging from approximately 100,000,000,000 hertz (0.1 Terahertz) to 10,000,000,000,000 (10 THz), although even higher frequencies may be included if the generation method is also used to produce frequencies in this range. The production of coherent THz radiation has proven to be challenging because the frequencies may be too high to be generated with microwave electronics. However, the frequencies may also be too low to use standard optical techniques and sources such as lasers. Until quantum cascade lasers were developed, it was difficult to build a THz laser with good transitions.
With the advent of ultrafast lasers, it was possible to generate few-cycle THz pulses. Such pulses have been used widely in connection with terahertz time domain spectroscopy (THz-TDS). The detection of the pulse can be coherent, for example with electro-optic crystals and a short probe pulse, thus enabling extraction of amplitude and phase information. This may be a powerful spectroscopic technique in applications associated with chemistry, biology and homeland security. However, it should be noted that THz generation by optical pulses may be inefficient. The power efficiency may be as small as 1×10−3 or lower.
Amplified lasers have been used to address power efficiency issues. Strong THz pulses may be produced with amplified lasers and low repetition rates. However, amplified lasers may be expensive. There is a need for a less expensive, practical source of THz pulses.
One method of producing ultrashort THz pulses was to illuminate a biased semiconductor with ultrashort optical pulses, produced, for example, by a mode-locked laser. The photocarriers may be accelerated by the electric field from the bias, and the changing current may result in radiation. Because the carriers may be produced on a sub-picosecond time scale, the radiated pulse may be referred to as a THz frequency pulse.
Some early prior art photoconductive emitters were referred to as Auston or photoconductive switches and were used as sources of ultrashort THz pulses. Another common early prior art technique of producing ultrashort THz pulses was using difference frequency generation (DFG), also known as optical rectification, in nonlinear media. Using the Auston switches, a single cycle pulse may be generated with no need for phase-matching. However, using a DFG technique, phase-matching may be necessary.
More recently, prior art photoconductive emitters include Ti:sapphire oscillator laser sources. These include a semi-insulating wafer of gallium-arsenide with attached electrodes composed of either metal or silver paint. The gap between the electrodes may range in width from a few microns to hundreds of microns or more. High voltage may be applied to the electrodes, either modulated in order to facilitate lock-in detection, or constant in order to get the THz average power.
Using a narrower gap between electrodes, the same field may be obtained using a smaller bias voltage. It has been found that the behavior of impurities in semi-insulating gallium arsenide (GaAs) results in electric field distribution that is different from that expected for a perfect dielectric. A good portion of the voltage drop may occur within a few microns of the positively charged electrode. Therefore, the THz power may depend strongly upon the location where the laser spot is focused with respect to the electrodes.
On the other hand, as disclosed in the prior art, a tightly focused laser spot may result in more ultrafast screening for a given laser power, thus limiting the THz emission. Pulses having a fluence larger than 50 microjoules per square centimeter (cm2) may not produce much more than weaker pulses in terahertz because of these screening effects. As the prior art has shown, these emitters use trap-enhanced fields, and the optimal beam shape may be elongated along the positive electrode so that screening effects are lessened. Trap-enhanced electric fields may make photoconductive switched extremely efficient at low incident optical power, but the THz generated may saturate rapidly with increasing excitation power.
For a beam having a waist below about twenty microns (20 μms), a key limitation on output power may be ultrafast screening of the applied field by photo-injected carriers. A line focus may be a desirable way of exciting these emitters since the carrier density may be lowered compared with a point focus, thus lessening the Coulomb screening. Large gains may be possible if the electric field is spread over a wider area of the sample. With high voltage bias and larger electrode spacing, high power “semi-large” area emitters have been achieved using semi-insulating gallium arsenide at about 100 MHz repetition rates.
There is a need for a terahertz generation system that does not incorporate emitters that use trap-enhanced fields. There is further a need for a terahertz generation system for which the electric field may be spread over a wider area of the sample.